The overall aim of the proposed research is to further analyze serologicaly the chomosome region in man which includes and surrounds the major histocompatibility locus (HL-A). The program will be directed 1) to extending the definition of the HL-A locus and 2) to detecting the lymphocyte defined (LD) non-HL-A loci by serologic means. A search will be instituted for human antisera which identify 1) new alleles at the HL-loci and 2) alleles governing the I region (MLR, Ir, GVHR) MLR-mixed lymphocyte culture reaction, Ir-immune response, and GVHR-graft versus host reactions expressed on the surfaces of thymus derived lymphocytes (T cells) and/or bone marrow derived lymphocytes (B cells). The interaction of the various serologically defined antigen systems will be correlated with disease susceptibility, with immune responses to various antigens and with the outcome of organ transplantation. BIBLIOGRAPHIC REFERENCES: Sasazuki, T., McMichael, A., Radvany, R., Payne, R. and McDevitt, H.: Use of High Dose X-Irradiation to Block Back Stimulation in the MLC Reaction, Tissue Antigen, 7:91, 1976. Calin, A., Fries, J., Schurman, D., and Payne, R.: The Close Correlation Between Symptoms and Disease Expression in HLA-B27 Positive Subjects, First Int. Symp. on HLA and Disease Abs., June, 1976. In press.